Conventionally, a digital camera is known which makes use of an output of a solid-state image pickup device to perform automatic exposure control, that is, to perform so-called imager AE.
For example, a digital camera is disclosed which includes an iris for controlling the amount of light to be admitted into a solid-state image pickup device and an image amplifier to which an image signal from the solid-state image pickup device is supplied and which assures the lowest image pickup subject illuminance of an average output level of the solid-state image pickup device irrespective of a variation of the iris (Japanese Patent Laid-open No. Hei 5-236338). In a digital camera of the type described above, an iris value is detected to correct the gain of the image amplifier.
In this manner, in a digital camera, the numerical aperture of the iris and the gain of the image signal are controlled to perform exposure control. Further, also by controlling the electronic shutter amount (shutter speed) of the image pickup device, exposure control can be performed similarly.
Incidentally, the operation mode of a digital camera upon image pickup successively changes to a monitoring mode, an image pickup preparation mode and an image pickup mode.
In particular, the monitoring mode is a mode in which, before the shutter button is depressed, an image of an image pickup subject is displayed on the display section in order to determine an image pickup range or composition. The numerical aperture of the iris in the monitoring mode is set to a value with which no smear occurs with the image pickup device.
The image pickup preparation mode is a state wherein the shutter button is half-depressed and a mode in which the three values of the numerical aperture of the iris, the electronic shutter amount and the gain of the image pickup signal are calculated and set based on the image pickup signal. In the image pickup preparation mode, the iris is driven so that the set numerical aperture may be established.
The image pickup mode is a state wherein the shutter button is depressed fully and a mode in which the electronic shutter of the image pickup device operates in a state wherein the opening of the iris, the electronic shutter amount and the gain of the image pickup signal all set in the image pickup preparation mode are established, and the image pickup signal is outputted from the image pickup device and image data are written into a memory.
In the conventional digital camera having such a configuration as described above, since the iris is formed as a mechanical element, the numerical aperture of the iris includes a hysteresis error and a dispersion error.
In particular, the hysteresis error is an error which appears, when the numerical aperture of the iris is set to a certain target value, between a numerical aperture in a case wherein the iris operates from the open side toward the smaller numerical aperture side and another numerical aperture in another case wherein the iris operates from the smaller numerical aperture side toward the open side.
Meanwhile, the dispersion error is an error of the numerical aperture of the iris which disperses within a certain range where the numerical aperture of the iris is set to a certain target value.
Therefore, in order to remove the hysteresis error described above, conventionally the iris is operated from one direction without fail such that the iris is set once to the open side after the image pickup mode is entered from the image pickup preparation mode, and then the iris is restricted to the target value.
The conventional digital camera configured so as to remove a hysteresis error of the iris in this manner requires operation time to restrict the iris to a target value after it is set once to the open side in the image pickup mode. Therefore, the conventional digital camera has a drawback that increased time is required, that is, a long shutter lag is involved, until the electronic shutter of the image pickup device operates to perform fetching of an image signal after the shutter button is depressed. Further, the dispersion of the numerical aperture of the iris which occurs when the iris is set to a target value cannot be removed, and this is disadvantageous when it is tried to improve the accuracy in exposure.
The present invention has been made in view of such a situation as described above, and it is an object of the present invention to provide a digital camera which is advantageous when it is tried to reduce the shutter lag and improve the accuracy in exposure.